Phosphate ion is an extremely important species in many fields. Phosphate is present in numerous biological systems, is a major constitutent of many minerals and fertilizers, and is a component of industrial wastewater. Phosphate is an important analyte, yet only one basic method exists for its assay.
The importance of orthophosphate concentration levels spans all areas of science and technology. A system that can continuously and selectively monitor phosphate levels in aqueous solutions will find numerous applications in fields such as pharmacology, biomedical research, clinical chemistry, industrial process monitoring, environmental monitoring, etc. Past attempts to develop a selective membrane electrode for phosphate have not been successful. Much of this past work has involved construction of electrodes based on membranes composed of various insoluble salts. In general, electrodes with either poor selectivity over common anions or impractically high detection limits have resulted from these previous attempts. A variety of liquid membranes and systems based on enzymatic reactions have also been investigated and found to be unsuitable for the selective determination of orthophosphate.
A series of extracting agents for phosphate and arsenate anions have been introduced. Long chain dialkyltin dinitrate species, such as dioctyltin dinitrate and didodecyltin dinitrate, are used to separate phosphate and arsenate from other anions by selective extraction into an organic layer. Liquid membrane electrodes employing these extraction agents have been demonstrated. Although it has been unsuccessful in developing phosphate selective polymer membrane electrodes with dialkyltin salts, it has been discovered that the incorporation of dibenzyltin dichloride derivatives into a plasticized PVC membrane gives a selective response to phosphate. Resulting electrodes possess practical detection limits and useful dynamic ranges of response.
Membrane electrodes containing dibenzyltin dichloride or bis(p-methylbenzyl)tin dichloride as active material have been disclosed in U.S. Pat. No. 4,735,692 which is incorporated herein by reference. Subsequent studies have shown that the response and selectivity characteristics of this known electrode were lacking in many respects.
Those concerned with these and other problems recognize the need for an improved phosphate selective membrane electrode.